Routing device optically switches data at gigabit rates

Routing device optically switches data at gigabit rates

Dave Wilson

Building on time-domain multiplexing technology, researchers at British Telecommunications Laboratories in Martlesham Heath, United Kingdom, have developed an all-optical high-speed routing device for fiber-optic networks. Built entirely of optical components, the routing device can switch data at speeds to 100 gigabits per second.

The routing device comprises two major circuits--an ultra-fast gate circuit based on a non-linear loop mirror process and a clock-pattern recovery circuit based on an optically frequency-modulated mode-locked erbium laser process. Working in tandem, or separately where required, the dual circuits optically direct time-domain multiplexed data through the routing device. In essence, the all-optical routing device enables system designers to multiplex several data channels. Data is delivered to networks via two output ports--one on the gate circuit and one on the clock circuit.

According to Julian Lucek, engineer at British Telecommunications Laboratories` network research unit, this type of optical-domain data processing has never been accomplished before. He adds, however, that the devices are still in the research and development phase and are not likely to be manufactured in volume or deployed in new systems for five years. "We are looking at 5- to 15-year time scales with projects like this," notes Lucek. He cites that the devices will probably not be used to upgrade existing networks, but would be best used in new systems built to handle 100 gigabits per second.

Routing process

Typically, the routing process in networks involves electronic devices that convert optical inputs into an electronic format. Then, electronic switching devices separate the data channels and deliver them to separate ports, where they are converted back to light by electronic devices.

Both the non-linear loop mirror gate and the optically, frequency-modulated mode-locked erbium laser were developed at British Telecommunications Laboratories. The erbium-doped fiber amplifiers, isolators and couplers are commercially available products.

British Telecommunications Laboratories operates fiber-optic networks at rates to 2.5 Gbits/sec. Theoretically, the routing device should allow system operation to 100 Gbits/sec. One possibility is a 100-Gbit/sec data stream comprising 10 channels, each running at 1 Gbit/sec.

An exclusive feature of the router device is remote programmability. The sequence of circuit settings is conducted by sending optical pulses across the system. The settings of the router device can be changed almost instantly because switching occurs in an optical format on fiber cable.

Optical fibers are not used to full capacity in telecommunications routing operations because electronic switches are relatively slow, complex and inflexible. The optical router device holds the promise of exploiting more fiber capacity.

Device operation

To describe routing device operation, consider the delivery of five optically time-division-multiplexed channels to the ultra-fast gate. This gate optically routes the first, second and fourth data channels to the clock-pattern recovery circuit; it routes the third and fifth data channels to the ultra-fast gate.

The ultra-fast gate delivers optical data bits to the clock circuit only when optical gating pulses are received. For example, a repeated 11010 control input pulse sequence is required by the gate to transmit the first, second and fourth data channels.

Data routed out the ultra-fast gate goes directly to the clock-pattern recovery circuit. Here, the data passes through the clock`s fiber-ring laser and out to the connected network. In passing through the clock circuit, the bits mode-lock the laser. As a result, the laser generates the gating pulses necessary to control the gate`s non-linear loop mirror by recovering an optical-clock signal from each data channel. The clock`s pattern output is fed back to the ultra-fast gate.

Lucek says that the ultra-fast gate contains an erbium-doped fiber amplifier that boosts the incoming data signal power to several milliwatts. The fiber amplifier, a general-purpose optical type commonly used at British Telecommunications Laboratories, provides gain in the 1510- to 1580-nanometer window.

The gate also incorporates two wavelength-dependent couplers. These couplers combine or split either the data wavelengths or the gating pulses, when necessary. The router data and the gating pulses run at 1514 and 1560 nm, respectively.

The clock-pattern recovery circuit is structured in a ring configuration. The circuit`s laser generates a clock pattern from the data received from the non-linear loop mirror. The data passes through dispersion-shifted fiber-optic cable that is shared by the laser cavity. Because a specific data pattern goes through the erbium-ring laser, the laser is forced to generate a pattern of clock pulses. This pattern is used to gate the non-linear loop mirror.q

Dave Wilson writes from London.